Method for the manufacture of vacuum tubes stems

Abstract

A method of manufacturing stems for vacuum tubes is disclosed. The method makes it possible, in particular, to improve the quality of the seal between the metallic bushings and the glass of the stem. The method consists in loading a bottom mold with a first glass piece and a second glass piece and metallic conductors, and then in heating the glass pieces before pressing them against each other between the bottom mold and the top mold. According to one characteristic of the invention, the first piece, which constitutes an external piece partially fixed in the bottom mold, is directly heated only on an upper part which is not fixed, and the two glass pieces are pressed against each other as soon as the upper part has reached a so-called working temperature in order to prevent prolonged contact between the upper part and a limited surface of the conductors.

What is claimed is: 
     
       1. A method for the manufacture of vacuum tube stems, said method comprising the steps of: (a) loading a bottom mold having an upper face with at least one metallic conductor, a first glass piece placed externally of and surrounding the at least one metallic conductor and extending above the upper face of the bottom mold, and a second glass piece placed internally of the first glass piece and so that the at least one metallic conductor is located between the first and second glass pieces;   (b) heating the portion of the first glass piece that extends above the upper face of the bottom molds; and   (c) pressing the first and second glass pieces against each other as soon as the upper portion of the first glass piece reaches the working temperature of the first glass piece, thereby preventing the formation of a crumbly oxide surface capable of breaking under the effect of mechanical or thermal stresses on the part of the at least one metallic conductor located between the first and second glass pieces.   
     
     
       2. A method as recited in claim 1 and further comprising the step of heating the second glass piece so that it has reached or exceeded the softening temperature of the second glass piece when the upper part of the first glass piece reaches its working temperature. 
     
     
       3. A method as recited in claim 1, wherein the at least one metallic conductor and the first and second glass pieces are heated without changing their relative positions with respect to the bottom mold. 
     
     
       4. A method as recited in claim 1 and comprising the further steps of: (a) removing the product obtained in step (c) of claim 1 from the bottom mold and then   (b) reheating the product to raise its temperature to the working temperature of the glass.   
     
     
       5. A method as recited in claim 1 wherein the upper portion of the first glass piece is heated by a burner. 
     
     
       6. A method as recited in claim 1 wherein the surface of the at least one metallic conductor is made of copper. 
     
     
       7. A method as recited in claim 1 wherein: (a) the first glass piece is a hollow cylinder prior to its heating and pressing;   (b) the second glass piece is a hollow cylinder prior to its heating and pressing;   (c) the first and second glass pieces are coaxial prior to their heating and pressing; and   (d) a plurality of metallic conductors are equiangularly spaced around the axis of the first and second glass pieces prior to their heating and pressing.   
     
     
       8. A method as recited in claim 1 wherein the upper end of the second glass piece is recessed beneath the upper end of the first glass piece. 
     
     
       9. A vacuum tube stem manufactured by the method of claim 1.